First of all, each of the girls can have a separate account using their own cc card. Just make sure that when they check-in at the port that they let the agent know this and provide both cc's.
As for pre-paying gratuities, that can be done pre-cruise through your travel agent OR once you get on board. You usually have until the day before the last day of the cruise to pre-pay gratuities at the customer service desk (your Daily Compass will have details).
I think the one word of advice that I would give a chaperone is to stress that they use a "buddy system". Let them know how important it is to not go off on their own in port (or onboard from the disco, etc.) A friend should be aware of their whereabouts at all times.